Laser irradiation of coal

Gases from Flash and Laser Irradiation of Coal... 

Gates from the flash and laser irradiation of Pittsburgh seam (hvab) coal were investigated to determine the action of high temperatures on coal. Temperatures in excess of 1000°C. were reached with both types of irradiation. Craters about 300 microns in diameter were produced in the coal with millisecond pulses from the laser unit rated at 1.7 joules output. Gaseous products from the laser and flash irradiations showed 21% and 8% acetylene, respectively. Diacetylene, vinylacetylene, and other products to molecular weight 130 were indicated in the mass spectrum of the gas from the laser study. The results indicated that the distributions of products obtained from the flash and laser irradiations of coal were different from that produced in high temperature carbonization. 

Table 15. Gaseous products from laser irradiation of coal in vacuum260)...

Table 15 compares the results of laser irradiation of coal with flash irradiation and conventional carbonization at 1200 K. Laser irradiation yielded higher conversions of coal and greater percentages of acetylene and higher hydrocarbons than flash irradiation or carbonization. When five bursts of 10 J energy were used to irradiate a cube of coal the acetylene yield increased from 20.9 to 25.9 mole % this constituted 90% of total hydrocarbon products. The decrease in H2 (Table 15), relative to the product from the flash irradiation, is possibly related to the increase in partially saturated structures such as ethylene (4.9%) and propylene (0.7%). Further, components with molecular weights up to 130 were found in the gas from laser irradiation diacetylene and vinylacetylene recognized as pyrolysis products of acetylene accounted for 2.4% of the product. 

In another series of experiments Friedel and coworkers268"270) studied the distribution of gaseous products from the laser irradiation of coals of various ranks and particle sizes in various atmospheres. In vacuum experiments the total gas yield varied inversely with coal rank, showing a four-fold increase between anthracite and lignite. The product gas composition as a function of volatile matter in coal is shown in Fig. 20. Yields of acetylene and hydrogen generally increased between anthracite... 

Fig. 20. Distribution of H2, CO, C02, CH4 and C2H2 in the products of laser irradiation of coal as a function of volatile matter in coal. (Redrawn from Karn, F. S., Friedel, R. A., Sharkey, Jr., A. G. Carbon 5, 25 (1967), by permission of the publishers, Pergamon Press Ltd.)...

Figure 2. Laser irradiation of Pittsburgh seam (hvab) coal, 84% carbon ...

Table IV. Hydrocarbons Produced by laser Irradiation of Pittsburgh Seam Coal in Vacuum ...

An attempt was made to use the volume of the craters to estimate the amount of coal irradiated and to relate this amount of coal to the gaseous product. It was difficult to determine the actual amount of material exposed to the laser beam. Some of the gas was probably produced in the peripheral region of the craters, and also there is evidence that a portion of the sample was simply blown from the crater. The yield of gaseous product based on the crater volume is given in Table I. 

We have heard several papers on the reactions of coal or coal volatiles under conditions of very high energy input (in plasmas, laser irradiation, etc.). Potentially useful yields of valuable chemical raw materials result. The work therefore appears to be a new variety of cook-book preparative chemistry in which new and highly active species are involved as intermediates, and it is not easy for the ordinary chemist to visualize what goes on and what these intermediates are. Would any of the authors care to comment ... 

Robert A. Friedel To Dr. Sharkey s remarks on mass spectroscopy I would like to add a few pertinent comments on other types of spectroscopy which would be applicable to the study of species obtained from coal subjected to plasma, laser irradiation, and similar high energy processes. 

Dr. Sharkey Attempts were made to use C H ratio following laser irradiation as an indication of the reaction temperature, as Dr. Linden suggests. Collecting the irradiated coal was difficult, and special techniques are now being devised. Use of a spectrum-line reversal technique is another possibility. 

In this review, in addition to more detailed experimental data for the synthesis of polyynes by laser ablation of graphite in solutions, new data for Indonesian and Australian coals are reported. In the experiments for graphite, besides benzene, toluene, and hexane used in our previous report [6], cyclohexane and methanol are used as solvents. It is known that long C2 F2 polyynes are generated by laser ablation of graphite in an F-containing gas [10]. Therefore, such F-containing solvents as hexafluorobenzene, perfluorooctane, and perfluorodecaline are also used as new solvents. The absorbances of polyynes in each solvent are measured not only at 355 nm laser irradiation reported previously [6] but also at 266, 532, and... 

FIGURE 7.7 HPLC chromatograms of Tanito Harum and Workworth coals at 355 and 532 nm laser irradiation in hexane. 

FIGURE 7.8 Absorbances of C H2 polyynes in hexane solution after 355, 532, and 1064nni laser irradiation on Tanito Harum and Workworth coals for 60 min. 

Lasers have been employed to produce extremely high tanperatures as has microwave heating of coal. Both techniques are reputed to produce considerable quantities of acetylene and the extent of the coal conversion depends on the volatile matter content of the coal, suggesting that the fixed carbon of the coal is not a predominant factor. Flash tubes with short (miCTOsecond or millisecond) irradiation times also produce acetylene as one of the major products and there appears to be little, if any, tar production. Furthermore, the presence of hydrogen appears to reduce the amount of acetylene produced. 

Table I. Gaseous Products from laser and Flash Irradiations and High Temperature Carbonization of Pittsburgh Seam (hvab) Coal...

When the concentrates of macerals of a high-volatile bituminous coal were irradiated with 6-J pulses from a ruby laser the total gas yield varied directly with volatile matter (13.4—55.4 rtiaf%) of the macerals273). Major gases evolved were H2, CO and C2 H2 their relative concentrations varied little among the macerals. 

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